Hi marcc79,
The 364 was used in Roamer`s military watches..check out Trim`s excellent Roamer site for info:
Military Roamers | Vintage Roamer Blog
You may be able to source a stem from Cousins U.K.as Mirius has previously suggested.
Trim`s website has loads of helpful technical PDF`s on the Roamer part numbers etc. which might help.
The crown style you will need will be different from those on his miltary Roamer`s blog page though as those have the waterproof case with a split stem.
Your watch is more likely to have had a much slimmer flatter style crown similar to none waterproof cased examples from the period featured in the Gallery section of Trim`s site.
I have quite a few Roamer`s from the early/mid `40`s with this style of crown
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Early MST 372 and MST 360
If the balance on your watch is good have you considered looking out for another better overall condition 364 and using yours a donor?
Sometimes good condition ones pop up cheaply as the balance can more easily get damaged on them due to lack of shock protection and balances can be more difficult to source- and replacing a balance staff extra expense.